Field of the Invention and Related Art Statement
The present invention relates to an optical pick-up comprising a light source means for emitting a light beam, beam collimating means for converting the light beam emitted from the light source means into a substantially parallel light beam, beam converging means for projecting the parallel light beam onto an optical information record carrier as a light spot, information being recorded as a spiral track or concentric tracks, and beam tilting means for tilting the light beam incident upon the converging means to move the light spot on the optical information record carrier in a direction substantially perpendicular to the information track to effect the tracking control.
FIGS. 1A and 1B are schematic views showing a known optical pick-up for recording information on an optical record carrier and reading the information out of the record carrier. A diverging light flux emitted by a semiconductor laser diode 21 and polarized in a given direction is converted into a parallel light beam by means of a collimator lens 22, and the parallel light beam is made incident upon a shaping prism 23 for correcting an intensity distribution in the light beam cross section. The light beam emanating from the shaping prism 23 is then made incident upon an objective lens 30 by means of a polarizing beam splitter 24, a quarter wavelength plate 25 and a swingable tracking mirror 26, and is projected onto an optical record carrier 28 as a fine light spot.
When the pick-up is used to read the information out of the record carrier 28, the light beam reflected by the optical record carrier 28 has been modulated by the information recorded on the record carrier, and is made incident upon the objective lens 27. The light beam emanating from the objective lens 27 is then made incident upon the polarizing beam splitter 24 by means of the swingable tracking mirror 26 and quarter wavelength plate 25. Since the light beam has passed through the quarter wavelength plate 25 twice, the light beam is reflected by the polarizing beam splitter 24. The light beam reflected by the polarizing beam splitter 24 is made incident upon a beam splitter 29 having a half mirror 29a. A light beam transmitted through the half mirror 29a of beam splitter 29 is collected by a collecting lens 30 and is made incident upon a first light detector 31 which produces an information signal picked-up from the optical record carrier 28. A light beam reflected by the half mirror 29a of beam splitter 29 is transmitted through a half wavelength plate 33 and is then made incident upon a detection prism 33 having a reflection plane Pr which is set to a critical angle of reflection with respect to the principal axis of the incident light beam. The light beam is made incident upon the reflection plane Pr as the P-polarized light and the light beam reflected by the plane Pr is made incident upon a second light detector 34 which is arranged at a far field of the objective lens 27.
As illustrated in FIG. 1C, the second light detector 34 has four light receiving regions a, b, c and d. As is well known in the art, the focusing error can be derived by (a+b)-(c+d) and the tracking error can be obtained by (a+c)-(b+d). By driving a focusing actuator in accordance with the focusing error it is possible to effect the focusing control, and by swinging or rotating the swingable tracking mirror 26 in accordance with the tracking error, it is possible to carry out the tracking control.
FIG. 2 is schematic view showing the manner of effecting the tracking control in the known optical pick-up. A pivot center P of the swingable mirror 26 is set at a cross point between the optical axis of the objective lens 27 and the light path along which the principal light ray passing through the optical axis of the collimator lens 22 travels. When there is no tracking error, the light ray passing through the center axis of the collimator lens 22 shown by a solid line is made incident upon the objective lens 27 in parallel with the optical axis thereof and is made incident upon the information carrier 28 perpendicularly thereto. The light ray reflected by the record carrier 28 passes along the same optical path as that along which the incident light ray is made incident upon the record carrier 28.
In order to effect the tracking control, the tracking mirror 26 is swung or rotated by an angle .theta. as shown by a broken line. Then the light beam projected onto the record carrier 28 is shifted by a distance .DELTA.x in the tracking direction. That is to say, when the tracking mirror 26 is rotated by the angle .theta., the light beam reflected by the tracking mirror is made incident upon the objective lens 27 with the incident angle of 2.theta. with respect to the optical axis of the objective lens 27. In other words, the light beam is made incident upon the objective lens 30 at a point O.sub.2 which is deviated from the center O.sub.1 by a distance D.sub.1 as shown by a broken line.
Now it is assumed that the objective lens 30 has a focal length of f, the light ray reflected by the mirror 29 is made incident upon the record carrier 28 at a point P.sub.2 which is deviated from the point P.sub.1 on the optical axis of the objective lens 27 by a distance of .DELTA.x=f.multidot..theta.. The light beam reflected by the record carrier 28 is collected by the objective lens 30 and is reflected by the swingable tracking mirror 26. This light beam is made in parallel with the incident light beam propagating along the optical axis, but is shifted therefrom by a distance D.sub.2.
In the known optical pick-up shown in FIG. 1A, when the mirror 26 is swung by the angle .theta., the light beam is made incident upon the objective lens 27 at the inclination angle of 2.theta. and is shifted by the distance D.sub.1 with respect to the center point O.sub.1 of the objective lens, so that the intensity distribution of the light spot formed on the record carrier 28 is deviated from the symmetrical pattern and the light beam reflected by the record carrier 28 is shifted from the incident light ray by the distance D.sub.2, so that the light spot formed on the second light detector 34 has the asymmetrical intensity distribution and is shifted in the tracking direction Tr. This results in that the tracking error signal contains undesired off-set components, and therefore the accuracy of the tracking servo control is decreased.
There have been proposed various methods for removing or mitigating the above mentioned off-set in the tracking error signal. For instance, in Japanese Patent Application Laid-open Publication Kokai Sho 51-22448, there is described a known optical pick-up in which the tracking mirror is arranged to swing about an axis which is on a cross line between a plane including the incident pupil of the objective lens and a plane including the tracking mirror, so that the light beam having the symmetrical intensity distribution is made incident upon the objective lens. In an actual pick-up, the mirror and swinging axis are coupled with each other by means of a resilient supporting member. However, when the swingable mirror is arranged under the focusing actuator, the resilient supporting member might interfere with the focusing actuator, and it is difficult to make a distance from the mirror to the objective lens longer owing to the substantial size of the resilient supporting member.
In Japanese Patent Application Laid-open Publication Kokai Sho 56-134325, there is described another known pick-up, in which the tracking mirror is supported by two springs having different spring constants, so that the mirror is swung as well as is moved linearly. However, in this known pick-up, it is rather difficult to set spring constants of these two springs, the point of the gravity of the mirror, the mirror driving position and so on to suitable values. Further, the sensitivity of the rotation of the tracking mirror is predominantly determined by a difference between the compression and expansion of the two springs, and thus it is very difficult to increase the sensitivity.
In Japanese Patent Publication No. 62-45614, there is disclosed still another known pick-up in which the swinging axis of the tracking mirror is positioned at the focal point of the objective lens. However, from the practical point of view, it is impossible to set the swinging axis at the focal point of the objective lens owing to the lens supporting member and focus driving coil. Moreover, since the position of the tracking mirror is fixed to the focal point of the objective lens, the freedom in designing the mirror arrangement is limited to a great extent.